The art of producing epitaxial layers of single crystal materials formed from compounds derived from elements of group IIIB of Hubbard's Chart of the Atoms, and elements selected from group VB thereof is well-known in the art. Such materials are customarily formed for various electrical applications including epitaxially filmed or layered semiconductors, diodes, amplifiers, transistors, solar cells, thermophotaic cells, micromodular circuits, rectifiers, thermoelectric generators, optical filters, watt meters and other semiconductor devices.
The films are often formed by well-known vapor deposition methods as for example described in U.S. Pat. No. 3,146,137 and in Solid State Electronics, Volume 8 (1965) pages 178-80 "The Preparation of High Purity Gallium Arsenide By Vapor Phrase Epitaxial Growth".
Gallium arsenide for example can be prepared as is known by bubbling hydrogen carrier gas through an arsenic trichloride bubbler with the resultant material passed into a quartz tube-type reactor over a gallium source at a first temperature and from there to a substrate on which is deposited gallium arsenide at a second temperature. Such vapor deposition is known in the art to produce epitaxial layers. Dopants in the form of gases can be passed into the reaction chamber so that various n and p layers of epitaxial growth can be obtained.
It has been difficult in prior art procedures to obtain epitaxial layer formation of a highly controlled nature with high production yields. Various changes in parameters and reaction conditions often affect yield production rates and commercial feasibility of producing certain devices.
Although it is known that a hydrogen carrier gas bubbled through the arsenic trichloride to form the reactor input gas should provide a constant concentration material for input in order to keep reaction conditions constant and obtain substantially constant rate of growth for the epitaxial layer, it has been difficult to do that. The prior art has attempted to bubble the hydrogen through arsenic trichloride at a constant temperature to obtain constant concentration output. However, as the liquid level of arsenic trichloride goes down in any reaction, the amount of cooling of the gases entering and leaving changes, making temperature control extremely difficult. Moreover, liquid cools as it evaporates and leaves the arsenic trichloride bubbler, again making temperature control difficult.
When dopants are added to the reaction area in order to obtain n or p doped epitaxial layers of growth, this can greatly affect the rate of growth of the epitaxial layers. If the concentration of the reactor vapor passed over the substrate substantially changes because of addition of a dopant, growth rate will also substantially change.